This invention relates to improvements in an apparatus for electron beam lithography and more particularly, to an apparatus for electron beam lithography capable of automatic focusing.
The manufacture of an LSI (large scale integration) or IC (integrated circuit) may employ a method wherein an electron beam accelerated to a high velocity is converged to a fine beam by an electron lens system and is simultaneously deflected and scanned in two dimensions by a scanning coil, so as to process the surface of an object into a microscopic pattern. In this case, with respect to the shape of the electron beam which impinges on a specimen such as of electron beam resist, there is a technique in which the image of an electron source is reduced as it is; that is, it is reduced to the size of a spot smaller than the precision of the pattern. Another technique includes uniformly projecting the electron beam onto an aperture plate having an opening (e.g. square or rectangular) and the resultant electron beam in the shape of the opening of the aperture plate is reduced. As compared with the former technique employing the spot beam or point beam, the latter technique has the advantage that since the specimen of a fixed area can be illuminated once, the processing time can be sharply reduced. In the latter technique, however, there are the disadvantages that the electron beam image from the aperture plate must be accurately projected on the specimen and that the detection of defocusing is difficult.
Regarding the conventional spot beam or point beam, the electron beam shape on the job surface, in the case of changing the focal point, varies as illustrated in FIG. 1A. The size of the beam spot becomes the smallest for correct focusing indicated at (3). In FIG. 1A, (1) and (2) illustrate cases of insufficient focus and (4) and (5) illustrate cases of excess focus. Accordingly, where a reference mark having a shape as shown by way of example in FIG. 2 and made of a substance (e.g. gold (Au)) differing in atomic number from the substance of a substrate (e.g. silicon (Si)) is applied on the work surface, where the electron beam is scanned orthogonally to the edge part of the reference mark and where reflected electrons, secondary electrons, or the like, thus generated are detected, an output as illustrated in FIG. 3A is obtained due to the difference of the reflection factors of the substrate and the mark for the electrons. Thus, correct focusing is indicated by the condition for which the gradient of an output waveform (1) becomes the most abrupt, that is, for which a waveform (2) for the differentiated values of the output waveform becomes the highest.
On the other hand, for a shaped beam having a square-like cross-section, the beam cross-section for changes in the focal point varies as illustrated at (1) to (5) in FIG. 1B. The beam size does not become the smallest at a correct focusing depicted at (3) in the Figure. Accordingly, an output waveform (1) obtained by scanning the edge part of the reference mark 2, as shown by way of example in FIG. 2, and a differentiated waveform (2) thereof become as in FIG. 3B. It is extremely difficult to know the condition of the correct focusing from these output waveforms. (1) and (2) in FIG. 1B illustrate cases of insufficient focus and (4) and (5) illustrate cases of excess focus.